Field ionization mass spectrometry is an attractive technique for the analysis of multicomponent mixtures due to the very low level of fragmentation for this type of ionization. This technique has been applied in our laboratory to the study of multicomponent mixtures from physiological fluids and shows good promise of diagnostic value. In order to obtain the fullest use of the information contained in metabolic profiles, one would ultimately like to identify the constituents at any particular molecular weight that are found to be statistically correlated with a given normal or pathological state. Field ionization alone is unable to provide this structural information. In order to retain the advantages of field ionization in obtaining the initial molecular ion profile, and at the same time acquire structural information on the individual components, we have chosen to combine the technique of field ionization and collision induced dissociation. The fragmentation pattern resulting from collisional excitation thus provides structural information on any molecular ion beam chosen for analysis.